Semiconductor emitting devices, such as light emitting diodes (LEDs) and laser diodes (LDs), include solid state emitting devices composed of group III-V semiconductors. A subset of group III-V semiconductors includes group III nitride alloys, which can include binary, ternary, and/or quaternary alloys of indium (In), aluminum (Al), and/or gallium (Ga) with nitrogen (N). Illustrative group III nitride based LEDs and LDs can be formed of materials in the form of InyAlxGa1-x-yN, where x and y indicate the molar fractions of a given element, 0≤x, y≤1, and 0≤x+y≤1. Other illustrative group III nitride based LEDs and LDs can include one or more layers formed of group III nitride materials including boron (B) nitride (BN), and can be of the form GazInyAlxB1-x-y-zN, where 0≤x, y, z≤1, and 0≤x+y+z≤1.
An LED is typically composed of a heterostructure of semiconducting layers. During operation of the LED, an applied bias across doped layers leads to injection of electrons and holes into an active layer where electron-hole recombination leads to light generation. Light is generated with generally uniform angular distribution and can escape the LED die by traversing semiconductor layers in all directions. Each semiconducting layer has a particular combination of molar fractions (e.g., x, y, and z) for the various elements, which influences the optical properties of the semiconducting layer. In particular, the refractive index and absorption characteristics of a semiconducting layer are sensitive to the molar fractions of the semiconductor alloy.
An interface between two semiconductor layers is defined as a semiconductor heterojunction. At the interface, the combination of molar fractions is assumed to change by a discrete amount. A semiconductor layer in which the combination of molar fractions changes continuously is said to be graded. Changes in molar fractions of semiconductor alloys can allow for band gap control, but can lead to abrupt changes in the optical properties of the materials and result in light trapping. A larger change in the index of refraction between the semiconductor layers, and between the substrate and its surroundings, results in a smaller total internal reflection (TIR) angle (provided that light travels from a high refractive index material to a material with a lower refractive index). A small TIR angle results in a large fraction of light rays reflecting from the interface boundaries, thereby leading to light trapping and subsequent absorption by semiconductor layers or LED metal contacts.
Roughness at an interface allows for partial alleviation of the light trapping by providing additional surfaces through which light can escape without totally internally reflecting from the interface. Nevertheless, light only can be partially transmitted through the interface, even if it does not undergo TIR, due to Fresnel losses. Fresnel losses are associated with light partially reflected at the interface for all the incident light angles. Optical properties of the materials on each side of the interface determine the magnitude of Fresnel losses, which can be a significant fraction of the transmitted light.
Semiconductor layers with graded composition are well known to result in polarization doping. A previous approach suggests to use polarization doping to form a p-n junction for semiconductor nanostructures. Similar suggestions also have been proposed in the past.